The use of seismic surveys, such as those obtained through vibratory surveys of subsurface geology, is fundamental in the prospecting for oil and gas reservoirs. As, known in the art, conventional seismic surveys of both the marine and land-based variety are based upon signals detected by arrays of receivers in response to many seismic "shots" imparted to the earth in the survey area. Dramatic improvements in the data acquisition and data processing technologies over recent years have made the generation of three-dimensional seismic surveys commonplace in the art, and have greatly improved the sensitivity and resolution of the surveys. These improvements have been necessitated by the inherent difficulty in finding those reservoirs of the world that have not been previously exploited.
After recording and storage of the detected seismic signals, conventional seismic signal processing techniques process and spatially arrange the data into a survey of the subsurface geology. Conventional techniques such as normal moveout, migration to correct for dip and diffraction effects, and noise filtering, are first applied to the seismic signals to remove known sources of error. Spatial arrangement of the corrected signals, using conventional techniques such as common midpoint gathers and stacks, generation of amplitude-versus-offset ("AVO") indicators, and the like, is then performed to result in a seismic survey (either two- or three-dimensional in nature) indicative of the subsurface geology in the survey region. Modern computing systems enable the handling of large volumes of signal data in the generation of these surveys, and also enable the human analyst to view three-dimensional surveys along any orientation.
After the seismic survey has been acquired, processed, and generated, the survey must be interpreted in order to fully understand the geology represented by the signal data. This interpretation, which is typically performed by a skilled analyst, generally includes the digitization of horizon surfaces within the three-dimensional survey volume. Horizon surfaces are surfaces that are selected, or numerically interpreted, to coincide with reflections in the volume represented by the seismic survey. The digitization of horizon surfaces can thus be considered to convert an arrangement of time-domain seismic signals into a graphic representation of the subsurface geology, in two or three dimensions. The depth, size, and locations of interfaces between geological formations can be deduced from such a representation, and used in guiding exploratory and production drilling, and in defining new survey techniques and arrangements. Of course, the accuracy with which the horizon surfaces are digitized or interpreted is critical in the success of such activities.
A first conventional method for interpretation of horizons from the seismic survey is commonly referred to in the art as direct manual picking. According to this approach, a planar "slice" in the three-dimensional survey volume is selected by the interpreter and is displayed, generally on an interactive computer system but also possibly by way of printed output (in more primitive systems). While the slice is typically made along one of the three orthogonal axes in the volume of line, crossline, and time, a slice may also be made along traverses not aligned with any one axis. In direct manual picking, the human analyst merely selects the location of horizon surfaces using his or her judgment, based upon the seismic signals themselves, and indicates the selection by activating a pointing device (mouse, trackball, etc.). While direct manual picking may be quite accurate when done by an experienced analyst, the sheer volume of seismic data in conventional three-dimensional surveys makes such an approach extremely slow, time-consuming, and costly.
By way of further background, an operation commonly referred to as "snapping" is conventionally applied to manually picked interpreted horizons. As is known in the art, hand-picked horizons are generally not precisely located on reflection events because of manual picking errors. The snapping operation is performed by moving the horizon along each trace from the initial manually picked position to a local maximum amplitude (or minimum amplitude, or zero crossing, as desired), thus adjusting the horizon to precisely match the reflection event being interpreted.
A conventional semi-automated approach to horizon interpretation is commonly referred to as autotracking, or volume autotracking. According to this technique, slices are again made in the seismic survey volume, and displayed by the computer system. The human interpreter selects "seed" points which he or she considers to be at a horizon surface, but need not pick an entire surface. After such selection, the computer system begins to extend horizon surfaces from the seed points, based on a selected algorithm and according to the seismic signals at neighboring locations, resulting in a connected surface extrapolation of the horizon. Volume autotracking is generally most successful when applied to surveys of relatively smooth and well-behaved geology, however, due to its requirement that the horizon surfaces remain connected; discontinuities, faults, reflection event splitting, and other ambiguities in the geology present problems to conventional volume autotracking systems.
A third conventional approach to horizon interpretation is referred to in the art as surface-slice interpretation. This approach is described in Stark, "Surface slices: Interpretation using surface segments instead of line segments", Expanded Abstracts of the 1991 Society of Exploration Geophysicists Annual Meeting, and in Stark, "Surface slice generation and interpretation: A review", The Leading Edge, Vol. 15, No. 7 (July 1996), pp. 818-819. Surface-slice interpretation is an automated approach in which the analyst selects a thin slab of the seismic volume, for example at a selected depth or time, in which the automated computer system identifies potential reflective events. For example, seismic signal amplitudes above a certain threshold may be identified as reflective events. Reflective events are then similarly identified in the next incremental slab in time or depth, and are "joined" to those reflective events in the previous slice that can be considered as part of the same horizon. A set of surfaces are thus generated through the repetition of this process; for example, an anticline would appear as a set of concentric shells. The surface-slice interpretation system is often referred to as "21/2-dimensional", due to its linking of events from two-dimensional slices. While the surface-slice interpretation approach is somewhat more efficient than the volume autotracking approach, this process can be time-consuming and difficult when the geologic structure is complex or when the seismic signal is weak. In addition, discontinuities and faults encountered in complex geology can also result in ambiguities when interpreted by the surface-slice method.
While the automated approaches of volume autotracking and surface-slice interpretation are quite efficient for certain geologies, each of these conventional techniques are quite slow in the interpretation of complex geologies, as, of course, is direct manual picking. In addition, conventional 3-D seismic surveys now generally involve huge volumes of data; for example, a typical modern survey may consist of on the order of 2000 shot lines, each shot line having 1500 traces per shot line, and each trace having 3000 time samples. As a result, these conventional manual or automated horizon interpretation techniques can be quite time-consuming, especially when applied to large surveys involving complex geologies. Since the prospecting for oil and gas reserves are now often concentrated in difficult and deep locations of the earth, considering that many of the shallow reservoirs have already been exploited and surveyed, there is an important need in the field for an efficient approach to horizon interpretation of seismic surveys.
It is therefore an object of the present invention to provide an automated system and method for performing horizon interpretation which can be efficiently applied to complex geological survey regions.
It is a further object of the present invention to provide such a system and method in which the human effort is efficiently applied to the interpretation process.
It is a further object of the present invention to provide such a system and method which performs the interpretation process in a robust manner when encountering discontinuities, faults, reflection event splitting (doublets), and other complexities in the surveyed geology.
It is a further object of the present invention to provide such a system and method which results in a full three-dimensional horizon interpretation when 3-D seismic data is available, but which is also applicable to 2-D seismic data, and to pre-stack seismic data.
It is a further object of the present invention to provide such a system and method which can readily handle large volumes of seismic data, such as those obtained from multiple 3-D surveys, either for adjacent regions or as repeat surveys.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.